1. Field of the Invention
The present invention relates to an automatic focusing device and an automatic focusing method for use in a camera or an imaging device capable of performing focus adjustment of the photographic lens. Focusing adjustment is accomplished by determining the defocus amount of the photographic lens from the light from the subject coming through the photographic lens.
2. Description of Related Art
An automatic focusing device is known for imaging devices such as cameras and video cameras in which focus adjustment of the photographic lens is performed by determining the amount of deviation (defocus amount) of the photographic lens from the light from the subject coming through the photographic lens. For instance, refer to Japanese unexamined patent publication Hei 2-34003.
With this type of automatic focusing device, the light from the subject coming through the photographic lens is introduced to an image sensor through a focus detection optical system. A pair of subject images are formed on the image sensor and converted into a pair of subject image data according to the light intensity distribution. On the basis of the pair of subject image data, the amount of defocus is detected. The amount of defocus comprises the difference between the image-forming plane of the subject image for the photographic lens and a predetermined focal plane that coincides with the film plane. The defocus amount is converted into the driving amount for the focusing lens included in the photographic lens through a predetermined conversion coefficient and a correction coefficient. Focusing is accomplished by driving the focusing lens according to the driving amount.
However, because there is a non-linear relationship between the defocus amount of the photographic lens and the driving amount of the focusing lens, it is troublesome to accurately convert the defocus amount into the actual driving amount of the focusing lens using a single conversion coefficient.
In particular, in recent times, as photographic lenses have become more compact, lighter weight and with higher variable magnification, the non-linearity of the relationship between the defocus amount and the driving amount of the focusing lens has increased. In addition to the quantitative increase in non-linearity, the factors involved in the conversion coefficient have also become more troublesome.
In actuality, with some types of photographic lenses, the ratio between the maximum and minimum values of the conversion coefficient over the entire focusing region can vary by more than a factor of three. This kind of large variance in the conversion coefficient includes many non-linear factors, making it impossible to accurately calculate the lens driving amount by merely correcting the conversion coefficient to be linear, as has conventionally been done.
In addition, conventional telephoto lenses have had essentially no variance in the conversion coefficient and have had highly linear properties. However, this linearity has been lost due to the recent trend toward zoom features. This kind of trend in telephoto lenses has had a large effect on automatic focusing devices. In other words, telephoto lenses have come to have a large amount of focus driving, causing the defocus amount to increase. In addition, they are frequently used in applications such as tracking a moving subject, so that a sophisticated automatic focusing function is required. When the lens focus driving amount is great, error in the driving amount of the focusing lens causes the generation of a large out-of-focus effect. Consequently, even from the perspective of the anticipated focusing function, it is necessary to reduce errors in the driving amount as much as possible.
When this kind of recent compact, light weight and high variable magnification photographic lens is driven by a conventional automatic focusing device, time is required for the focusing to take place. Further, the focusing lens may move beyond the focusing point or stop before reaching the focusing point. All of these problems result in an out-of-focus photograph. In addition, with respect to moving subjects, the focusing lens does not converge to the focusing position, causing the occurrence of the "hunching" phenomenon.
In order to solve these kinds of problems, consideration was given to improving the method of correcting the conversion coefficient used to convert the defocus amount into a lens driving amount. Simply making the correction term a higher order term until the conversion error between the defocus amount and the lens driving amount is sufficiently small can also be considered. However, in order to do so, it is necessary to store large quantities of conversion coefficient correction data in the photographic lens corresponding to the magnification rate and focusing lens position. In addition, a large time interval is required for communicating and processing the data when conversion is made from the defocus amount to the lens driving amount, causing the focus adjustment responsiveness and convergence of the focusing lens to worsen.